SONET SDH Presentation

12002-02-119PMH P. Michael Henderson, michael.henderson@cox.net

Fundamentals of Fundamentals of SONET/SDHSONET/SDH

P. Michael Hendersonmichael.henderson@cox.net

AgendaWhy SONET/SDH?Historical backgroundBasic structure of the SONET/SDH frameTransport overheadPayload pointer processingPayload overheadMapping of SONET payloads− Virtual tributaries− Handling ATM, POS, and GFP

Automatic protection switchingSummary

This presentation is based on a white paper available at http://members.cox.net/michael.hendersontitled “Fundamentals of SONET/SDH.”

What is SONET/SDH

SONET – Synchronous Optical NETwork (ANSI).− ANSI started work on SONET in1985.

SDH – Synchronous Digital Hierarchy (ITU).− The ITU began work in 1986 to achieve the same goal.

SONET and SDH define a set of physical layer standards for communications over optical fiber.

I will attempt to cover both SONET and SDH in this presentation.However, SONET and SDH use different terminology, which makes it difficult to talk about both at the same time.

I will talk mainly about SONET because you need to understand SONET to understand why some things were done in SDH.

Why SONET/SDH?

Originally, all communications in the telephone network was analog.Analog lines or analog microwave was used to connect to switching offices.

Switching office Switching office

Many individual analog lines

In about 1962, the network providers began using digital communications between switching centers.In the US, this was the D1 channel banks and T-carrier systems.

Two pairs of copper wires (Tx and Rx)

Channelbank

Analog lines Analog lines

Why SONET/SDH?

As communications needs grew, many T- or E-carrier lines were needed between switching centers.In the late 1970’s optical communications began to be used to interconnect switching offices.

Two optical fibers (Tx and Rx)

Terminating multiplexer

T1 lines T1 lines

Terminating multiplexer

Why SONET/SDH?

Standardization of SONET/SDH

Prior to standardization, every manufacturer of optical communications used their own framing.− Did not allow multi-vendor networks.

The ANSI T1X1.5 committee began work in 1985 to define standards for optical communications which would allow “a mid-span meet”.The ITU began work in 1986 to achieve the same goal.Both bodies finalized their first set of standards in 1988.

Why do we need SONET/SDH

All data requires framing.Since optical networks are complex, provisions are made for management information.Many other things are provided.− Multiplexing.− Error checking.− Handling variations in clocks.− Mapping of plesiochronous voice and data traffic.− Signaling for automatic switching in case of a fiber or node

failure.

SONET/SDH Terminology

Line Line

Section Section Section Section

STELTE

STEPTE

Regenerators RegeneratorsADM ADM ADM

PathTermination

SectionTermination

LineTermination

PTE = Path Terminating EquipmentLTE = Line terminating EquipmentSTE = Section Terminating EquipmentADM = Add/Drop Multiplexer

9 Rows

90 Columns

3 Columns oftransport OH

A1 A2 J0

B1 E1 F1

D1 D2 D3

H1 H2 H3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

Section overhead

Line overhead

Payload overhead

A1/A2 = 0xf628

STS-1 SONET Frame One column ofpayload OH

Synchronous Payload Envelope (SPE) – 87 columns

Order oftransmission

STS-1 Frame Format

9 rows by 90 columns – 810 octets in the frame.Frame is transmitted from left to right, by row.Frames are transmitted 8,000 times per second, every 125 µseconds.STS-1 bit rate is therefore 51.84 Mbps (810 octets x 8,000 times per second x 8 bits per octet).This lowest level SONET signal is called a Synchronous Transport Signal, level 1 (STS-1). Once the scrambler is applied, it is known as an Optical Channel, level 1 (OC-1).The lowest level SDH signal is know as a Synchronous Transport Module, level 1 (STM-1).

SONET/SDH data rates

SONET name SDH name Line rate (Mbps) SPE rate (Mbps) Overhead rate (Mbps)

OC-1 STM-0 51.84 50.112 1.728

OC-3 STM-1 155.52 150.336 5.184

OC-12 STM-4 622.08 601.344 20.736

OC-48 STM-16 2,488.32 2,405.376 82.944

OC-192 STM-64 9,953.28 9,621.504 331.776

OC-768 STM-256 39,813.12 38,486.016 1,327.104

A1 A2 J0

B1 E1 F1

D1 D2 D3

H1 H2 H3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

Z1 Z2 E2

A1 A2 J0

B1 E1 F1

D1 D2 D3

H1 H2 H3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

Z1 Z2 E2

A1 A2 J0

B1 E1 F1

D1 D2 D3

H1 H2 H3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

Interleaving of SONET/SDH signals

A1 A2 J0

B1 E1 F1

D1 D2 D3

H1 H1 H1

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

First STS-1

A1 A1 A2 A2 Z0 Z0

H2 H2 H2 H3 H3 H3

X X X X X X

B2 B2 X X X X

X X X X X X

Z1 Z1 Z2 M2 X

Second STS-1 Third STS-1

Interleaving of SONET/SDH signals

270 Columns

A1 A2 J0B1 E1 F1D1 D2 D3H1 H2 H3B2 K1 K2D4 D5 D6D7 D8 D9

D10 D11 D12Z1 Z2 E2

D10 D11 D12S1 M0/1 E2

J1B3C2G1F2H4Z3Z4Z5

J1B3C2G1F2H4Z3Z4N1

Payloads in Interleaved Signals

87 Columns

(N/3) – 1 Columns of fixed stuff

Concatenated Payloads

261 Columns

J1B3C2G1F2H4Z3Z4N1

Concatenated PayloadsA1 A2 J0B1 E1 F1D1 D2 D3H1 H2 H3B2 K1 K2D4 D5 D6D7 D8 D9

D10 D11 D12Z1 Z2 E2

D10 D11 D12S1 M0/1 E2 261 Columns

A1 A2 J0

B1 E1 F1

D1 D2 D3

H1 H2 H3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

Framing and Section Trace

Two octets used for framing: A1, A2.Bit pattern is A1 = 1111 0110, A2 = 0010 1000Higher levels of STS-N have N A1 octets and N A2 octets. Section Trace (J0) is used to verify continued connection between section entities.

A1 A2 J0

B1 E1 F1

D1 D2 D3

H1 H2 H3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

Monitoring for Bit Errors

Does a bit interleaved parity (BIP-8) over the octets in the (previous) frame.Even parity.Parity check applies to previous frame Separate check for section and line.Only one B1 octet, no matter what the STS-N.One B2 octet for each STS-N.

A1 A2 J0

B1 E1 F1

D1 D2 D3

H1 H2 H3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

Communication Channels

E1, E2 (orderwire) was intended to be used by craftspersons while installing a line. Craftspeople use cellular phones so this octet is not used very much.F1 is available for the network provider to use as they wish.

A1 A2 J0

B1 E1 F1

D1 D2 D3

H1 H2 H3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

Communication Channels

These channels are used for network management.Technically, D1, D2, D3 is intended for section messages but this is not always adhered to.

A1 A2 J0

B1 E1 F1

D1 D2 D3

H1 H2 H3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

Automatic Protection Switching

These octets (K1, K2) are used to send messages between two nodes when a failure is detected between them.Messages are sent both ways around the ring (if possible).Last four bits of K1 specifies the address of the addressed node.First four bits of K2 specifies the address of the sending node.Means that there can only be 16 nodes on a ring.

A1 A2 J0

B1 E1 F1

D1 D2 D3

H1 H2 H3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

Sync and Error Indication

S1 is used for synchronization status.M0/M1 is used to send back to the sender, the error status of the received signal (determined by the Bx octets).

A1 A2 J0

B1 E1 F1

D1 D2 D3

H1 H2 H3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

Payload Pointers

The H1, H2 octets are joined to form a pointer which points to the first octet of the SPE.The first 4 bits of the 16 bits indicates if the pointer is changing.The next two bits are not used.The last 10 bits are the pointer and can have a value from zero to 782.H3 is used to carry a payload octet when a negative pointer adjustment is done.These octets are covered in additional detail in the next slides.

1,000,000 bits/sec

Network box which takes traffic from one side and passes it along to the other side

1,000,001 bits/secFIFO

Why do we need Payload Pointers?

Every eight seconds, the FIFO will run dry. If an extra, “meaningless” octet can be sent at that time, it will give the FIFO time to fill up again.Problem: How to tell the receiver that this one octet is meaningless?

Format of the H1, H2, H3 Octets

N N N N – – I D I D I D I D I D

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

01 10 0 0 10 BIT POINTER

NEGATIVE STUFF OPPORTUNITY

POSITIVE STUFF

OPPORTUNITY

H2H1 H3

NNNN = New data flag (NDF)I = Increment bitsD = Decrement bits

H1 H2 H3

A Negative Pointer Adjustment

H1 H2 H3

Normal Frame

“D” bits inverted.

Pointer updated.

Normal Frame

The pointer is invalid here because of the “D” bit inversion. The value without inversion is the same as the previousframe.

This line has no meaning –it’s just there to help show the movement of the start of the frame (here).

Note that H3 is being used to carry a payload octet

New Data Flag I0 0 0

Unused D1 1 X X 0 0

1 1 X X

1 01 1 1 1 1I I I ID D D D

Normal frameInvert “D” BitsNew ptr valueNew ptr valueNormal frame

Frame status

0 01 1

0 1 0 0 11 0 1 0 1

0 0 0 1 11 1 1 1 0

Negative Justification using Inverted “D” Bits

Details of H1, H2, H3 octets during justification

A Positive Pointer Adjustment

H1 H2 H3

Normal Frame

“I” bits inverted.

Pointer updated.

Normal Frame

The pointer is invalid here because of the “I” bit inversion. The value without inversion is the same as the previousframe.

Positive stuff octet

This line has no meaning –it’s just there to help show the movement of the start of the frame (here).

Unused D1 1 X X 0 0

1 1 X X

1 01 1 1 1 1I I I ID D D D

Normal frameInvert “I” BitsNew ptr valueNew ptr valueNormal frame

Frame status

0 01 1

1 0 1 1 00 1 0 1 0

0 0 0 1 11 1 1 1 1

Positive Justification using Inverted “I” Bits

Details of H1, H2, H3 octets during justification

Unused D1 1 X X 0 0

0 0 X X

1 01 1 1 1 1I I I ID D D D

Normal frameNDF indicatorNew ptr valueNew ptr valueNormal frame

Frame status

0 01 1

0 0 0 1 11 1 1 1 0

One Octet Negative Adjustment using the NDF

Details of H1, H2, H3 octets when using NDF

Unused D1 1 X X 0 0

0 0 X X

1 01 1 1 1 1I I I ID D D D

Normal frameNDF indicatorNew ptr valueNew ptr valueNormal frame

Frame status

0 01 1

0 0 0 1 11 1 1 1 1

One Octet Positive Adjustment using the NDF

Details of H1, H2, H3 octets when using NDF

Payload Overhead

Path trace (J1) used to check continued connection between the path devices (end-to-end).64 octet for SONET and 16 octet for SDH message.

Error Checking

BIP-8 parity check over the payload, only.Even parity.Parity is for previous payload, not this one.

Path Signal Label

Indicates the type of payload carried in the SPE.For our discussion, we’ll be interested in a value of 0x02 for floating virtual tributary (VT) mode, 0x04 for asynchronous mapping of DS-3, 0x13 for mapping of ATM, 0x16 for packet over SONET (POS), and 0x1b for generic framing procedure (GFP).

Path Status and Path User Channel

The G1 octet (Path Status) sends information back to the sender indicating the number of parity errors, or if a complete failure is detected.The F2 octet is a user channel for Path applications (end-to-end). It is not subject to standardization (anything can be put into it).

Multi-Frame Indicator

The last two bits of this octet counts from 00 to 11 continuously to provide a multi-frame indicator for VT payloads (to be explained later).Some of the other bits are being defined for use with virtual concatenation but will not be described here.

TCM and Reserved octets

The Z3, Z4 octets are reserved for future standardization and have no meaning today.The N1 octet is for tandem connection monitoring and is fairly complex. No attempt will be made here to explain it.

Virtual Tributaries

Plesiochronous Data Rates of Interest

Type of Digital Circuit

Bit Rate (Mbps)

DS-1 (T1) 1.544

E1 2.048

DS-1C 3.152

DS-2 6.312

DS-3 (T3) 44.736

87 ColumnsPayloadoverhead(POH)

STS-1 Payload Mapping

All mappings of payload into an STS-1 payload have these two columns taken as stuff columns.

28 Cols 28 Cols 28 Cols

Virtual Tributary Groups

The 84 usable payload columns are divided into seven groups of twelve columns. Each set of twelve is called a “Virtual Tributary Group” (VTG)

When interleaving,the first columnof VTG#1 goes next to the POH.Columns 30 & 59 of the SPE are skipped.

The “x” in Vx goes from 1 to 4

Virtual TributariesA Virtual Tributary Group is further subdivided into Virtual Tributaries (VT).− Three columns makes a VT-1.5 (1.728 Mbps gross, good for a

DS-1 at 1.544 Mbps).− Four columns makes a VT-2 (2.304 Mbps gross, good for an E1

at 2.048 Mbps).− Six columns makes a VT-3 (3.456 Mbps gross, good for a DS-

1C at 3.152 Mbps).− Twelve columns makes a VT-6 (6.912 Mbps gross, good for a

DS-2 at 6.312 Mbps).A VTG contains four VT-1.5s, or three VT-2s, or two VT-3s, or one VT-6.A VTG can only contain one kind of VT. Different VTGs in an SPE can contain different VT types, but within a VTG, there can only be one kind of VT.

Virtual Tributaries – The VT-1.5

The four VT-1.5s are interleaved into the VTG. Note the colors – there are three of each color, with each color indicating one VT-1.5 (for four inside the VTG. Each VT is 27 octets, with one octet taken for the Vx octet.

Vx Vx Vx Vx

One VTG

One VT-1.5 ++

If all the VTGs in an STS-1 are carrying VT-1.5s, the capacity of the STS-1 SPE is 28 DS-1s, or the same as a DS-3. (Seven VTGs times four VT-1.5s per VTG)

SuperframesRemember that the last two bits of H4 in the POH count 00, 01, 10, 11, 00, etc. This produces a superframe of four frames.The frame after the SPE with an H4 value of 00 will have the first octet in the VT identified as V1. The one after value 01 will be identified as V2, etc.V1 and V2 form a pointer, exactly like the H1, H2 octets.

Bits 5 and 6 are used to indicate the type of VT (unused in H1, H2) (VT-6=00, VT-3=01, VT-2=10, VT-1.5=11)

V3 is the negative stuff opportunity, exactly like H3.V4 is reserved for future standardization.

V1 V2 V3 V4

The V1, V2 Pointers

N N N N S S I D I D I D I D I D

V1 V2V1

0 1 1 0 0 010 BIT POINTERVT6

0 1 1 0 0 1 10 BIT POINTERVT3

0 1 1 0 0110 BIT POINTERVT2

1VT1.5

NEW DATA FLAG – INVERT 4 N BITS NEGATIVE STUFF – INVERT 5 D BITS POSITIVE STUFF – INVERT 5 I BITS

I – Increment Bit D – Decrement Bit N – New Data Flag BIt S - VT Size Bit

Pointing to the Start of the VT Payload

V1 V2 V3

Pointer value of zero

Pointer value of 103

A VT-1.5 is made up of three columns of 9 octets or 27 octets. Four frames make a supreframe, or 108 octets. Once the V1, V2, V3, V4 octets are removed from the count, we have 104 octets remaining.Since the pointer counts from zero, the highest value of the pointer is 103.

Frame 1 Frame 2 Frame 3 Frame 4

26 octets

V5 J2 Z6 Z7

The VT-1.5 Payload

Byte Synchronous and Asynchronous VTs

We’re going to look at two ways that DS-1 traffic can be carried in a VT payload – byte synchronous and asynchronous.Byte synchronous preserves the location of the payload octets in a T1 frame (each speech sample).− Used primarily to transport channelized T1s which are carrying

voice calls.Asynchronous simply transports the 1.544 Mbps stream without concern for which byte is which.− Used to carry T1s which are carrying data.

Bits 5, 6, & 7 of V5 indicate which is being carried (010 = asynchronous, 100 = byte synchronous).

26 octets

V5 J2 Z6 Z7P1P0S1S2S3S4FR P1P0S1S2S3S4FR P1P0S1S2S3S4FR P1P0S1S2S3S4FR

24 DS-0Channels

Byte Synchronous Mapping – VT-1.5

R = Fixed stuff bitF = DS-1 framing bitSx = Signaling bits, A, B, C, D bits in a DS-1Px = Phase bits, indicates which frame set for SF or ESF framing

26 octets

V5 J2 Z6 Z7R R R R R R I R C1C2O O O O I R C1C2R R R S1S2R

24 Octets(192 bits)

C1C2O O O O I R

Asynchronous Mapping – VT-1.5

R = Stuff bits – no meaningO = Future standardization – unusedI = Information bit. Makes the 193rd bit of

a DS-1 frame

Cx = Controls use of the S bits.Sx = Stuff bits. May or may not carry

information bits, depending upon values in Cx

24 Octets(192 bits)

24 Octets192 bits)

Finished with Virtual Tributaries!!!

Support for DS-3 signals

A DS-3 signal at 44.736 Mbps takes the whole STS-1 SPE.− No virtual tributaries are used.

A DS-3 can only be carried with asynchronous mapping.− There is no byte synchronous mapping for a DS-3.

There are 77 full information payload octets, or 616 information bits.The C1 octet has 5 information bits, making the total number of information bits per row 621.Nine of these per frame, times 8,000 frames per second gives 44.712 Mbps.− So how do we carry a 44.736 Mbps signal in 44.712 Mbps?

The answer is in the “s” bit in C3. Sometimes it carries an information bit.− If it carried an information bit every frame, the data rate would

be 44.784 Mbps.

Support for ATM, POS, and GFP

Asynchronous Transfer Mode (ATM), Packet over SONET (POS) and Generic Framing Procedure (GFP) are simply mapped into the SPE as a serial data stream, octet aligned with the SONET/SDH octets.When traffic is mapped into an STS-1 payload, columns 30 and 59 are not used for payload (fixed stuff).ATM, POS and GFP can be mapped into higher speed concatenated payloads.− Mapping is simply done by putting the ATM, POS or GFP octets

into the concatenated SPE. SONET/SDH does not examine the payload octets in any way.

Generic Framing ProcedureA way of framing variable length data without a framing character.− To overcome the problem of “shielding” in POS.

GFP is being defined in the ANSI T1X1.5 committee.− Can be thought of a variable length ATM type of framing.− Header has header check octets and payload length. GFP

frame delineation is similar to finding ATM header.

PLI = Payload length indicatorHEC = Header error control (CRC-16)

PLIPLI

HECHEC

Variable lengthpayload

4 octets

Length specified in the two octets of the PLI is for total frame length, including header. Max frame size, therefore, is (64K-1) octets (65,535 octets).

Automatic Protection Switching

ADM B sends traffic directly to ADM A. But ADM A has to send traffic for ADM B all the way around the ring.

A Unidirectional Ring

Working fiber

Protection fiber

Backup for a Unidirectional Ring

Traffic between ADM A and ADM B flows bi-directionally between them.

A Bi-directional Ring

Working fibers

Protection fibers

Backup for a Bi-directional Ring

Path Switching

Today, path switching is only used on unidirectional rings –hence the name Unidirectional Path Switched Rings (UPSR).What does Path Switching mean?− At the exit node, both fibers are monitored and the path traffic

extracted.− Based on several factors, especially error rate, the “best” traffic

is selected to be handed off to the customer.To a large degree, path switching can be done completely by the receiver, without coordination with the sender.But, path switching requires two sets of electronics to extract the path information.

Add/Drop Multiplexer

Working fiber

Protection fiber

Traffic, e.g.,DS-1, E1, DS-3, STS-1, etc.

Path Switching – How does it work?

Line Switching Line Switching is done on bi-directional rings, either two fiber or four fiber. Thus the name Bi-directional Line Switched Ring (BLSR). The total capacity of the fibers must be twice the carried traffic. For four fiber systems, this means two fibers are dedicated for protection.What does Line Switching mean?− Two adjacent nodes monitor the traffic between them.− If one detects “failure” on a fiber, it signals the other.− The two nodes coordinate switching to the protection fiber(s).

The K1, K2 octets in the transport overhead are used for this signaling.

ADM B(detail afterring switch)

4 3 2 1

ADM A(detail afterring switch)

Line Switching – How is it done?

Working fibers

Protection fibers

Summary

SONET/SDH is a complex subject, with many layers.

This presentation has attempted to take you from the SONET/SDH frame to the virtual tributaries.

There’s a lot more to SONET/SDH than was covered in this presentation – many things were ignored to avoid hopeless complexity.

I hope this introduction will stimulate some of you to study the subject in more detail, perhaps by reading the standards.

Questions?

Backup Material

Carrying an E1 in a Virtual Tributary

Virtual Tributaries – The VT-2

The three VT-2s are interleaved into the VTG. Note the colors – there are four of each color, with each color indicating one VT-2 (for three inside the VTG. Each VT is 36 octets, with one octet taken for the Vx octet.

Vx Vx Vx

One VTG

One VT-2+++

SuperframesRemember that the last two bits of H4 in the POH count 00, 01, 10, 11, 00, etc. This produces a superframe of four frames.The frame after the SPE with an H4 value of 00 will have the first octet in the VT identified as V1. The one after value 01 will be identified as V2, etc.V1 and V2 form a pointer, exactly like the H1, H2 octets.

Bits 5 and 6 are used to indicate the type of VT (unused in H1, H2) (VT-6=00, VT-3=01, VT-2=10, VT-1.5=11)

V3 is the negative stuff opportunity, exactly like H3.V4 is reserved for future standardization.

V1 V2 V3 V4

The V1, V2 Pointers

N N N N S S I D I D I D I D I D

V1 V2V1

0 1 1 0 0 010 BIT POINTERVT6

0 1 1 0 0 1 10 BIT POINTERVT3

0 1 1 0 0110 BIT POINTERVT2

1VT1.5

NEW DATA FLAG – INVERT 4 N BITS NEGATIVE STUFF – INVERT 5 D BITS POSITIVE STUFF – INVERT 5 I BITS

I – Increment Bit D – Decrement Bit N – New Data Flag BIt S - VT Size Bit

Pointing to the Start of the VT Payload

V1 V2 V3

Pointer value of zero

Pointer value of 139

A VT-2 is made up of four columns of 9 octets or 36 octets. Four frames make a supreframe, or 144 octets. Once the V1, V2, V3, V4 octets are removed from the count, we have 140 octets remaining.Since the pointer counts from zero, the highest value of the pointer is 139.

35 octets

V5 J2 Z6 Z7

The VT-2 Payload

Byte Synchronous and Asynchronous VTs

We’re going to look at two ways that E-1 traffic can be carried in a VT payload – byte synchronous and asynchronous.Byte synchronous preserves the location of the payload octets in an E1 frame (each speech sample).− Used primarily to transport channelized E1s which are carrying

voice calls.Asynchronous simply transports the 2.048 Mbps stream without concern for which byte is which.− Used to carry E1s which are carrying data.

Bits 5, 6, & 7 of V5 indicate which is being carried (010 = asynchronous, 100 = byte synchronous).

35 octets

V5 J2 Z6 Z7

Byte Synchronous Mapping – 30 Channels

R = Fixed stuff bitR = may be used for channel 0, if requiredPx = Phase bits, indicates which channel the CAS bits apply to

P1P0R R R R R R

Channels1 - 15

R R R R R R R R

Channels16 - 30

P1P0R R R R R R

Channels1 - 15

R R R R R R R R

Channels16 - 30

P1P0R R R R R R

Channels1 - 15

R R R R R R R R

Channels16 - 30

P1P0R R R R R R

Channels1 - 15

R R R R R R R R

Channels16 - 30

35 octets

V5 J2 Z6 Z7

Byte Synchronous Mapping – 31 Channels

R = Fixed stuff bitR = Indicates may be used for channel 0

R R R R R R R R

Channels1 - 15

R R R R R R R R

Channels16 - 30

Channel 16

R R R R R R R R

Channels1 - 15

R R R R R R R R

Channels16 - 30

R R R R R R R R

Channels1 - 15

R R R R R R R R

Channels16 - 30

R R R R R R R R

Channels1 - 15

R R R R R R R R

Channels16 - 30

Channel 16 Channel 16 Channel 16

35 octets

V5 J2 Z6 Z7

Asynchronous Mapping – VT-2

I = Information bit (Data)R = Fixed stuff bitO = Overhead – same as R

R R R R R R R R

32 octets

R R R R R R R R R R R R R R R R

C1C2O O O O R R

32 octets

R R R R R R R R

32 octets

R R R R R R R R

C1C2O O O O R R C1C2R R R R R S1

S2 I I I I I I I

31 octets

Cx = Stuff controlSx = Stuff opportunity

Synchronous Digital Hierarchy (SDH)

SDH Terminology

SDH terminology is different from SONET and several new concepts are introduced.One primary difficulty is that SDH starts with the equivalent ofan STS-3 signal but has to map the same PDH signals.SDH makes much more use of the “group” terminology.− Compare to Virtual Tributary Groups (VTGs) in SONET.

Let’s look at the STM-1 frame and the ways that PDH signals get mapped.

Administrative Unit Groups and AUs

An Administrative Unit Group (AUG) consists of the payload portion of the frame, plus row four of the overhead.SONET has no equivalent of the AUG.The AUG can contain either:− Three Administrative Unit level 3 (AU-3), or− One AU-4.

We’ll look at the AU-3 first and then at the AU-4.

Section overhead

Administrative Unit Group (AUG)

270 Columns261 Columns

B1 E1 F1

D1 D2 D3

H1 H1 H1

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

A1 A1 A2 A2 Z0 Z0

H2 H2 H2 H3 H3 H3

X X X X X X

B2 B2 X X X X

X X X X X X

Z1 Z1 Z2 M2 X

The STM-1 frame (not to scale)

Administrative Unit - 3

The AUG is larger than needed to carry PDH traffic. Plus, some technique must be provided to maintain some compatibility with SONET STS-1.Three AU-3s fit inside the STM-1 AUG.Begins to look a lot like an STS-3 carrying three STS-1 signals.

B1 E1 F1

D1 D2 D3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

A1 A1 A2 A2 Z0 Z0

X X X X X X

B2 B2 X X X X

X X X X X X

Z1 Z1 Z2 M2 X

H3H2H1H3H2H1

H3H2H1

87 Columns

Section overhead

Administrative Unit level 3 (AU-3)

Mapping of three AU-3s to an AUG

But the AU-3 doesn’t “float” like the payload of an STS-1 signal. How do we accommodate clock differences?We use a structure known as a Virtual Container – 3 (VC-3).− The AU-3 has fixed stuff columns at columns 30 and 59, just like

SONET.− The VC-3 does not include these fixed stuff columns – they’re

taken into account when the VC-3 is mapped into the AU-3.− So the VC-3 has only 85 columns – 84 columns of payload and

one column of payload overhead.

J1B3C2G1F2H4Z3Z4N1

H3H2H1

87 Columns

85 Columns

The “Floating” Virtual Container - 3

Mapping PDH Traffic When mapping PDH traffic, the VC-3 will contain seven Tributary Unit Group – 2s (TUG-2). These function exactly like the VTGs in SONET.Then, within the TUG-2, we have Tributary Units (TU-x) which carry the traffic exactly like SONET. The names are different.− Actually, each TU contains a Virtual Container which carries the

traffic. See next slide for convention.

TU-2/VC-2VT-6TU-12/VC-12VT-2TU-11/VC-11VT-1.5SDH NameSONET Name

SDH Naming Conventions

Level 1 is the lowest level structure. Numbers get larger as the structure gets larger.Traffic is always carried in a container.A container becomes a Virtual Container (VC) with the addition of Path Overhead (POH)A Tributary Unit (TU) is a Virtual Container (VC) plus a pointer.A TU fits into a Tributary Unit Group (TUG).When a structure fills another structure, it gets the same number. Example, a TU-2 fills a TUG-2. A VC-3 fills an AU-3.

An alternate to the AU-3 is the AU-4.The AU-4 structure takes the entire payload of an STM-1− 261 columns plus the pointers in row four of the section overhead.

Problem: How to map the PDH traffic in this larger frame?Hint: If we can get to a VC-3 structure, the TU structure will be exactly as described for the AU-3.

B1 E1 F1

D1 D2 D3

B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M0/1 E2

A1 A1 A2 A2 Z0 Z0

X X X X X X

B2 B2 X X X X

X X X X X X

Z1 Z1 Z2 M2 X

261 Columns

Section overhead

Administrative Unit level 4 (AU-4)

H1H1 H1 H2H2 H2 H3H3 H3

Tributary Unit Group - 3

Define a new structure known as the Tributary Unit Group – 3 (TUG-3).Consists of 86 columns, one larger than the 85 columns of a VC-3.The extra columns carries pointers to allow the VC-3 to “float.”Three 86 column TUG-3s will take 258 columns, leaving three columns in the 261 column AU-4.One column is used for the payload overhead (POH) just like all other payloads.The two columns behind the POH are fixed stuff.So three TUG-3s fill an AU-4.

VC-3VC-3

86 ColumnsFi

85 Columns

VC-3 Mapping into a TUG-3

Virtual Container - 3

Now that we have reached the VC-3 structure, all of the Tributary Unit mappings are the same as described earlier.

Added note: There’s another terminology in the TUG-3 case which I skipped for simplicity. The combination of the VC-3 plus the H1, H2, H3 pointers in the TUG-3 are called a Tributary Unit level 3 (TU-3). This is similar to the way the AU-3 consists of the payload plus the H1, H2, H3 pointers from the AUG.

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